Absolute humidity sensors and methods of manufacturing humidity sensors

ABSTRACT

Disclosed are humidity sensor structures, and fabrication techniques, which result in uniform and reliable humidity sensing, reliable electrical connections in small sensors, and simplified and inexpensive manufacture.

This is a division, of application Ser. No. 764,394, filed Jan. 31, 1977now U.S. Pat. No. 4,143,171.

BACKGROUND OF THE INVENTION

The present invention relates to humidity sensor structures and toimproved fabrication techniques for manufacturing such structures.

Commercially acceptable absolute humidity sensors have been known forsome time. A particularly successful sensor is described and claimed inGoodman et al. U.S. Pat. No. 3,523,244, owned by the assignee of thepresent invention and incorporated herein by reference. The structuretaught in that patent consists of an aluminum foil which is anodized toform a porous Al₂ O₃ layer on one surface of the foil. A thin, moisturepermeable gold layer is then deposited over the porous Al₂ O₃ toproduce, essentially, a parallel plate impedance that is sensitive tothe water vapor that can easily penetrate the thin gold layer. Theoverlying gold layer and the underlying aluminum foil form the parallelplates of the impedance. Electrical contact to the thin gold film can bemade in a number of ways, one of which utilizes a spring-loaded metalfinger that makes mechanical contact with the gold film.

Despite the substantial success of the absolute humidity sensordescribed in U.S. Pat. No. 3,523,244, its design has dictated stringentfabrication requirements. Thus, it is a principal object of the presentinvention to provide an improved absolute humidity sensor design whichis conducive to relatively inexpensive fabrication and accuratemeasurement. Another object is the provision of fabrication methodswhich will yield accurate humidity sensors efficiently andinexpensively.

SUMMARY OF THE INVENTION

Briefly, the invention herein features humidity sensor structures, andtechniques of fabrication, which provide very uniform surfaces and layerthicknesses, as well as reliable electrical connection between variouselements of the sensor, in order to achieve the objects set forth above.Thus, in one aspect, the invention features the method of manufacturinga humidity sensor that comprises the steps of: (a) providing anon-metallic substrate having a microscopically smooth surface, (b)building up a uniform layer of Al on the substrate surface, (c) formingan oxide on at least a major portion of the area of the Al layer toprovide a porous Al₂ O₃ layer over Al layer, (d) building up anelectrically conductive layer, which is substantially permeable to watervapor, over at least a major portion of the Al₂ O₃ layer, and (e)building up an electrically conductive strip in contact with thepermeable electrically conductive layer and extending beyond a border ofthe Al₂ O₃ layer, without electrical contact with the underlying Al, toan electrical contact location on the substrate.

In another preferred method of fabrication of a humidity sensor, themethod comprises the steps of: (a) providing a silicon substrate havinga microscopically smooth surface, (b) masking a region of the surface,(c) growing a layer of non-porous SiO₂ on a region of the Si surfacesurrounding the masked region, (d) unmasking the region, (e) providing alayer of porous SiO₂ in that region, (f) building up an electricallyconductive layer, which is substantially permeable to water vapor, overat least a major portion of the exposed surface of the porous SiO₂, and(g) providing means for establishing electrical contact with the layerproduced in step (f) and with the Si substrate beneath the porous SiO₂layer.

The invention also features humidity sensors, and absolute humiditysensors in particular, constructed in accordance with these generalmethods to provide: a substrate having a microscopically smooth surface;a planar, porous, dielectric non-conductive first layer of materialhaving a first face bonded to the substrate surface and a second face,said faces separated by a distance of the order of 2500 A or less; meansestablishing electrical contact with the first face; and meansestablishing electrical contact with the second face. The latter meansfor contacting said second face may be formed of a layer of moisturepermeable, electrically conductive material bonded to the second faceand an electrically conductive strip bonded to the conductive layer andextending beyond the boundaries of the non-conductive layer to a contactlocation formed on said substrate.

In preferred embodiments of either of the above methods and of theresulting structures, a thin film or diffused heater and a temperaturesensor (e.g., thin film or PN junction) can be provided on the substrateacent to, or layered with, the actual humidity sensor in order toprovide the capacity for operation of the humidity sensor at atemperature above the ambient.

As will be apparent from the discussion below, other details offabrication and structure form features of various preferred embodimentsof the present invention and contribute, in addition to the generalmethods and structures discussed above, to the achievement of theobjects set out above.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features, and advantages of the invention will appearfrom the following description of particular preferred embodiments andtechniques thereof. In the drawings, thicknesses and other dimensions ofthe various elements have been exaggerated for clarity. In the drawings:

FIG. 1 is a plan view of one preferred embodiment of an absolutehumidity sensor constructed in accordance with the principles of thepresent invention;

FIG. 2 is a view taken at 2--2 of FIG. 1;

FIGS. 3 and 4 are views similar to FIGS. 1 and 2 of an alternativeembodiment;

FIGS. 5A, 5B and 5C illustrate steps in the fabrication of still anotherembodiment of an absolute humidity sensor in accordance with the presentinvention;

FIG. 6 is a plan view of an absolute humidity sensor includingtemperature sensing means and heating means;

FIG. 7 is a view similar to FIG. 6 of an alternative embodiment;

FIGS. 8A and 8B are side elevations illustrating alternative techniquesfor preparing an absolute humidity sensor such as is shown in FIG. 7;

FIG. 9 illustrates a step in the manufacture of humidity sensorsincorporating features of the present invention; and

FIGS. 10A, 10B, 11A and 11B are schematic diagrams illustrating combinedheating and temperature sensing arrangements for humidity sensorsconstructed in accordance with the present invention.

DETAILED DESCRIPTION OF PARTICULAR PREFERRED EMBODIMENTS

FIGS. 1 and 2 illustrate a first example of an improved absolutehumidity sensor formed on a substrate 10. The substrate consists of achip of silicon 12 upon which a thermally grown or deposited layer 14 ofSiO₂ is provided. While precise dimensions of the chip are notimportant, a typical chip would be a square having 100 mil. sides. Thelayer 14 is prepared using conventional techniques to provide amicroscopically smooth upper surface 16. In the embodiment of FIGS. 1and 2, the actual humidity sensor is formed on the surface 16.

In forming the absolute humidity sensor, aluminum is deposited (e.g.,vacuum deposition, sputtering, or other appropriate methods) on thesuitably masked surface 16. The masking is such that the aluminum layeris formed to have a large central region 18, a lobe 20 at one side ofthe central region 18 and connected thereto by a tongue 22, and anisolated lobe 24 on the opposite side of the main portion 18 from thelobe 20. The lobes, which will serve as bonding pads, are built up to athickness not required of the region 16 in order to be rugged enough fortheir intended purpose. An oxide is formed on the exposed aluminumsurface of region 18, using any suitable conventional technique, toprovide a layer 26 of Al₂ O₃ having a thickness of about 2500 A or less.(As taught in U.S. Pat. No. 3,523,244, such a thin porous layer is keyto achieving a true absolute humidity sensor. Other sensors, however,having thicker porous layers, can be improved by applying the principlesof the present invention.) Examples of techniques for anodizing aluminummay be found in Choo et al., "Barrier-Type Aluminum Oxide Films FormedUnder Prolonged Anodizing," J. Electrochem, Soc.: Solid-State Scienceand Technology, Dec., 1975, p. 1645 and Neufeld and Ali, "The Influenceof Anions on the Structure of Porous Anodic Al₂ O₃ Films Grown inAlkaline Electrolytes," J. Electrochem, Soc.: Electrochemical Scienceand Technology, April, 1973, p. 479.

After suitable further masking, an electrically conductive layer 28(e.g., gold) is built up (e.g., deposited) in a pattern to overlie amajor portion of the Al₂ O₃ layer 26. A strip extends from layer 28beyond the boundaries of layer 26 to overlap the isolated aluminum lobe24 to complete fabrication of the sensor. If required to assureelectrical contact between the central portion of gold layer 28 and thebonding pad 24, bridging fingers 30 are deposited over the gold layerextending between the lobe 24 and the central region 18. In a typicalconstruction, the gold layer 28 will have a thickness of approximately100 A to 500 A while the bridging fingers will have a thickness ofapproximately 2000 A or more.

Typically, a group of sensors as illustrated in FIGS. 1 and 2 will bemanufactured simultaneously. FIG. 9 illustrates steps in thesimultaneous manufacture of nine such sensors. An aluminum layer isdeposited on the upper surface of an oxidized silicon wafer 13 exceptfor a series of masked regions 15. The masked regions 15 cooperate todefine an array of nine rectangular regions 18 which are to serve as thelower electrodes for nine absolute humidity sensors as illustrated inFIGS. 1 and 2. A series of aluminum tabs 17 interconnect rectangularregions 18 with the aluminum layer around the periphery of the wafer 13and also interconnect the regions 18 with each other. The tabs 17 assurethat all aluminum coated portions of the wafer 13 are interconnected sothat a single oxide-forming electrode in electrical contact with aportion of the aluminum coating can accomplish anodizing of each of theregions 18. A series of scribe lines 19 are indicated in FIG. 9. Thelines 19 define the locations of future scribing after the remainingfabrication steps (e.g., deposition of upper, gold electrodes). Thescribing severs the wafer 13 and provides nine individual silicon chips,each supporting an absolute humidity sensor as illustrated in FIGS. 1and 2. The scribing, of course, severs the tabs 17, as indicated inFIG. 1. (The tabs 17 could be avoided by anodizing the individualaluminum electrode regions 18 separately using a multiple-pin contactarrangement that provides separate electrical connection for each of theregions 18.)

As will be apparent to those skilled in the art, the sensor illustratedin FIGS. 1 and 2, as well as the technique of fabrication justdescribed, will result in a porous Al₂ O₃ layer 26 having very uniformthickness sandwiched between the lower layer 18 of aluminum and theupper layer 28 of gold. These latter two layers serve as electrodes. Thelobe 20 serves as a bonding pad for making electrical contact with thealuminum electrode and the lobe 24 serves as a bonding pad forelectrical contact with the gold electrode.

Although with suitable layer thicknesses the bridging fingers 30 mightnot be required, they may be useful in assuring electrical contactbetween the gold layer 28 overlying the Al₂ O₃ and the bonding pad 24.For example, the gold layer will typically have a thickness of about 100A to about 500 A. Since the combined thicknesses of the aluminum and Al₂O₃ layers may be greater than 2500 A there is a likelihood that the goldlayer will be broken as it crosses the step from the upper surface ofthe Al₂ O₃ to the surface 16, a step many times its own thickness. Theheavy bridging fingers 30, which can be gold, aluminum, or any othersuitable conductive material, assure electrical contact between the goldlayer 28 and the bonding pad 24 while masking only a tiny fraction ofthe upper surface of the gold layer.

As will be apparent to those skilled in the art, the structureillustrated in FIGS. 1 and 2 simplifies fabrication, relative toprevious absolute humidity sensor designs. In particular, using thetechniques of fabrication described, the Al₂ O₃ layer will be extremelyuniform in thickness and electrical contact with the overlying goldelectrode can be easily accomplished by employing an integral gold layerstrip extending to the isolated bonding pad 24; and the bridging fingers30, if necessary.

The humidity sensor illustrated in FIGS. 1 and 2 provides an extremelyinexpensive and accurate humidity sensor for operation and storage atmoderate temperatures. At high temperatures (e.g., above 200° C.),additional oxidation of the aluminum layer 18 might cause shifting ofcharacteristics of the sensor with attendant inaccurate readings. FIGS.3 and 4 illustrate an example of an absolute humidity design thatfacilitates the same inexpensive and efficient manufacture as the designof FIGS. 1 and 2, but that is not as susceptible to high temperatureshifts. The absolute humidity sensor remains essentially a layer 32 ofAl₂ O₃, which is very thin and of uniform thickness, sandwiched betweena pair of electrodes. As with the previous embodiment, the upperelectrode is preferably provided in the form of a thin film of gold 34deposited over the layer 32 and including a strip extending beyond theboundaries thereof to a bonding pad 36. Bridging fingers 30 can beprovided if necessary. The lower electrode, however, is simply thesilicon chip 38 itself. For adequate electrical conductivitycharacteristics, the silicon is a low resistivity P-type silicon. Abonding pad 40 for this lower electrode is provided in the form of aheavy deposition of chromium-gold, or other electrically conductivemetal, in contact with the low resistivity silicon. The sandwich layers32, 34, as well as the bonding pad 40, are provided in "wells" etched ina layer 42 of SiO₂, which is grown on the microscopically smooth surface44 of the silicon chip 38.

The device of FIGS. 3 and 4 can be fabricated by the general techniquesdescribed above in relation to FIG. 9. A thin (e.g., 2500 A or less)layer of aluminum is deposited in a central well making good electricaland mechanical contact with the microscopically smooth surface 44 of thelow resistivity silicon chip 38. After this step, an oxide is formedthroughout the full volume of the aluminum, using any conventionaltechnique, to form the layer 32 of Al₂ O₃. (The process typicallyresults in a slightly thicker oxide layer--e.g., 1250 A when theoriginal aluminum thickness was 1000 A.) The use of the chip 38 itselfas the lower electrode and the complete oxidation of the aluminum duringfabrication, of course, contribute to the high temperature stability ofthe absolute humidity sensor, since there is no residual aluminum whichcan become oxidized during operation or storage at high temperatures.

After the formation of the Al₂ O₃, chromium-gold may be deposited toform the bonding pads 40 and 36. The thin (e.g., 100 A to 500 A) goldlayer 34 is deposited over a majority of the exposed surface of thelayer 32 and extending beyond the periphery of the layer 32 to overlapthe bonding pad 36. As mentioned above in connection with the embodimentof FIGS. 1 and 2, if the dimensions of the varous layers are such thatbreakage of the thin gold layer 34 may be likely, the bridging fingers30 can be formed as a final step.

The fabrication of another absolute humidity sensor capable of hightemperature applications is illustrated in FIGS. 5A-C. Once again a lowresistivity silicon chip 38 forms a substrate. The chip is thermallyoxidized to form a conventional insulating non-porous layer 46 of SiO₂,which may have a thickness of between 3000 A and 10,000 A. A window 48is then etched in the layer 46 to provide an exposed smooth surface 50of the silicon chip 38 which can receive the porous central layer of thethree layer absolute humidity sensor. In the embodiment underconsideration, this porous layer can be provided by forming an oxide ofthe silicon itself (e.g., in a boric acid solution) to provide a layer52 of porous SiO₂ which fills the window 48. Suitable techniques to forma porous region have been discussed in the literature (e.g., Cook,"Anodizing Silicon is Economical Way to Isolate IC Elements,"Electronics, November 13, 1975; Watanabe, et al., "Formation andProperties of Porous Silicon and its Application", J. Electrochem, Soc.:Solid-State Science and Technology, October 1975 ).

The porous layer 52 is then covered with a thin, permeable layer 54 ofgold to form the top electrode. Electrical contacts with the siliconchip 38 (which is, of course, the bottom electrode) can be obtained fromthe back side of the chip, after removal of any SiO₂ which may haveformed during steps described above. Alternatively, a top side contactcan be provided by etching a contact window 56 through the peripheralnon-porous SiO₂ layer 46 and depositing a heavy metal bonding pad 58 tofill the window 56 and provide contact with the silicon chip 38. At thesame time, a top electrode bonding pad 60 can be deposited over aportion of the gold layer 54 that extends beyond the layer 52 andoverlies the peripheral non-porous SiO₂ layer 46.

As will be evident to those skilled in the art, since the absolutehumidity sensor illustrated in FIG. 5C includes no free aluminum, thisabsolute humidity sensor will be capable of high temperatureapplication.

For sensors used as high quality absolute humidity sensors, the porousdielectric layer (the sensing element) has a thickness no greater thanabout 2500 A, in accordance with the teachings of the above-mentionedU.S. Pat. No. 3,523,244. With the improved techniques and structuresdisclosed herein, however, the sensor thickness may be reducedsubstantially below 2500 A (e.g., 1000 A or less). Such thinner porouslayers may be desirable for certain humidity sensing situations (e.g.,fast response to humidity changes). The accurate measurement of suchthin layers or films has been the subject of considerable study over theyears and various techniques have been developed. As will be realized bythose skilled in the art, these techniques include the use of verysensitive step-sensing styluses, optical techniques, weight measurementsof layer constituents, etc. For example, using the weight-basedtechniques, an absolute measurement of weight per unit area is obtained.From such measurements the layer thickness can be calculated. It hasbeen calculated that an Al₂ O₃ layer which is 2500 A thick correspondsto a weight of aluminum oxide of about 0.0001 grams per squarecentimeter. (The weight measurement techniques typically involve"backscattering spectrometry" and are discussed, for example, in Nicoletet al. "Backscattering Spectrometry", American Laboratory, March, 1975,p. 22; and in Mayer et al., "Thin Films and Solid-Phase Reactions," 190Science 228 (17 Oct. 1975).)

As is known to those skilled in the art, in various applications it maybe desirable to operate an absolute humidity sensor at a temperatureother than the ambient (e.g., elevated operating temperatures may allowfaster response at high moisture levels, prevent condensation, andinsure repeatable conditions). To achieve a stable elevated temperature,a heater is provided which receives heating current from a control thatis responsive to a temperature sensor located near the humidity sensor.A suitable control, of course, can be of any conventional design. FIGS.6-8 illustrate heater and temperature sensor arrangements which areparticularly desirable for use with absolute humidity sensorsconstructed in the manner described above.

Referring first to FIG. 6, an absolute humidity sensor 62 of a typedescribed above can be provided at the center of the face of a wafersubstrate 64. After the absolute humidity sensor 62 has been fabricated(or even intermediate certain fabrications steps), a thin filmtemperature sensor 66 is deposited on the exposed surface of thesubstrate 64 in the form of a narrow strip encircling the humiditysensor 62. Heavier bonding pads 68 of a construction similar to thebonding pads described with reference to FIGS. 1 through 5 may beprovided for establishing electrical contact with the temperaturesensor. Also surrounding the humidity sensor is a deposited thin filmstrip heater 70 having heavier bonding pads 72. Suitable materials forthese elements are nickel or platinum for the temperature sensor 66 andnickel-chromium for the heater 70. With leads secured to the bondingpads 68 and 72, a conventional control mechanism can employ the read-outfrom the temperature sensor to control the current delivered to theheater thereby maintaining the local environment of the absolutehumidity sensor 62 at any desired temperature above the ambient.

In the embodiment of FIG. 7, again a temperature sensor 66 is depositedaround the periphery of the humidity sensor 62. In this embodiment,however, the heater is provided beneath the humidity sensor 62 in theform of a strip 74 of electrically resistive material. A layer ofdeposited SiO₂, or other insulation, insulates the heater from thesensor. Bonding pads 76 penetrate the peripheral insulating SiO₂ layerof the substrate 64 to provide electrical contact with the strip 74.

Two alternative constructions of the device illustrated in FIG. 7 areshown in the sectional views of FIGS. 8A and 8B. Referring first to FIG.8A, the substrate 64 comprises a silicon chip 80 having an insulatingSiO₂ layer 82 on one surface thereof. A suitable heater material (e.g.,nickel-chromium) is deposited on the surface of layer 82 to form thestrip heater 74. An insulating layer 84 is then deposited over the stripheater 74 and the humidity sensor 62 is fabricated on the exposedsurface 86 of the oxide layer 84, as in the thin film temperature sensor66. Wells are etched in the oxide layer 84 and the bonding pads 76 aredeposited in those wells for contact with the heater strip 74.

In the embodiment of FIG. 8B, the heater is not formed as a film ofdeposited metal, but rather by diffusing a dopant in the desired patternto form a P-type silicon resistor, formed in the shape, on the exposedsurface of a N-type silicon chip 88 which defines the substrate 64. Aswith the embodiment of FIG. 8A, an insulating layer 84 is deposited overthis heater arrangement and the remaining steps of fabrication follow.

In addition to the arrangements illustrated in FIGS. 7, 8A and 8B, it ispossible to employ the heater element itself as a temperature sensor,thus eliminating structure, and fabrication steps, that may be requiredin the embodiment of FIGS. 7-8B. Specifically, with the diffused heaterarrangement of FIG. 8B, it is proposed that the diffused resistor be"P-type" and that the resulting silicon PN junction can be employed as atemperature sensor. The electrical properties of a PN junction are suchthat temperature can be determined by measuring either the reverseleakage current through the junction or the forward voltage drop acrossthe junction at constant current. The physical structure would be quitesimilar to that illustrated in FIG. 8B, with the elimination of theseparate temperature sensor 66.

Schematic diagrams illustrating the operation of two alternatearrangements for temperature sensing employing the PN junction areillustrated in FIGS. 10A and 10B. In the embodiment of FIG. 10A, thevoltage drop across the PN junction in a forward biased condition ismeasured at a constant current. The voltage drop varies in a knownmanner with temperature and thus can be used as a measure oftemperature. Point A (corresponding to a bonding pad 76 of FIG. 8B) ismaintained at -5 volts and point B (the other pad) is modulated between-5 volts and -15 volts in accordance with the heating requirements. Thesubstrate (point C in FIG. 10A) is maintained at ground potential. Tomeasure temperature, the heater supply voltage is interruptedperiodically (e.g., once every second) and the forward voltage drop ofthe temperature sensor diode PN junction 90 operated at constant currentis measured through terminals A and C. This temperature dependentvoltage is used as the sensing signal in a closed loop temperaturecontrol circuit to maintain the substrate (and its moisture sensor) at aparticular temperature.

In the embodiment of FIG. 10B, a terminal D is provided for temperaturesensing function which is separate from the diffused heater 74. While inthis embodiment four lead wires are needed instead of the three leadwires of FIG. 10A, the temperature sensor can be operated on acontinuous basis, as opposed to the periodic mode described withrelation to FIG. 10A.

A modification of the arrangements illustrated in FIGS. 10A and 10B canprovide an accurate absolute temperature measurement and one that isrelatively insensitive to changes in the manufacturing procedure. Thismodification involves the provision of two PN junction diode temperaturesensors. Embodiments corresponding to FIGS. 10A and 10B are illustratedin FIGS. 11A and 11B, respectively.

By providing two diodes 92, 94 on the same substrate at a giventemperature having equal areas, the difference in the forward voltagedrop across these diodes is given by:

    V.sub.1 -V.sub.2 =ΔV=(nkT/q)ln(I.sub.1 /I.sub.2),

where V₁ is the forward voltage across the diode 1; V₂ is forwardvoltage across diode 2; n is a constant approximately equal to unity;"k" is Boltzman's constant; T is the absolute temperature; "q" is theelectronic charge; I₁ is forward constant current through diode 1; andI₂ is forward constant current through diode 2. Thus, the temperature isdirectly proportional to the voltage difference and inverselyproportional to the natural logarithm of the ratio of the two constantcurrents.

In the embodiment of FIG. 11A, the diffused heater 74 is provided with atwo terminal centertap defining terminals A' and B'. During the heatingmode, A' and B' are shorted. During the measurement mode, A' and B' areconnected to A and B, respectively, and the diodes are forward biased tospecific constant currents I₁ and I₂. Measurement of V₁ -V₂ then permitscalculation of the absolute temperature. In FIG. 11B (as in FIG. 10B),the heating and temperature sensing functions have been separated sothat each can operate in a continuous mode.

As will be apparent to those skilled in the art, the various absolutehumidity sensor arrangements described above are compatible withexisting micro-electronic technology and can therefore be convenientlyincorporated into integrated circuit structures. Furthermore, as theoperation of the absolute humidity sensor is presently understood, thedesigns according to the present invention are suitable for themanufacture of sensors having very thin porous sensing layers and thusare thought to be more sensitive to moisture at the low dew point regionthan are other known absolute humidity sensors.

While particular preferred embodiments illustrating the principles ofthe present invention have been shown in the accompanying drawings anddescribed in detail herein, other embodiments are within the scope ofthe invention as defined in the claims.

What is claimed is:
 1. The method of manufacturing a humidity sensorcomprising the steps of(a) providing an Si substrate having amicroscopically smooth surface,(b) forming a layer of non-porous SiO₂ ona first region of said Si surface surrounding a second region, (c)forming an oxide of the Si in said second region to provide a layer ofporous SiO₂, (d) building up an electrically conductive layer, which issubstantially permeable to water vapor, over at least a major portion ofthe exposed surface of said porous SiO₂, and (e) establishing electricalcontact with the layer produced in step (d) and with the Si substratebeneath said porous SiO₂ layer.
 2. The method as claimed in claim 1wherein said step (d) comprises the deposition of metal upon the exposedsurface of said porous SiO₂.
 3. The method as claimed in claim 2 whereinsaid step (e) comprises the deposition of an electrically conductivestrip extending from said metal layer to a contact location on saidnon-porous SiO₂ layer.
 4. The method as claimed in claim 3 wherein saidelectrically conductive strip is formed by deposition simultaneouslywith the formation of said metal layer.
 5. A humidity sensor constructedaccording to the method of claim 1.